1. Field of the Invention
The present invention relates to an improved method of evaluating acoustic anisotropy of construction materials, and an apparatus for performing such a task. It is especially useful for ultrasonic inspection of welded joints in architectural or civil engineering structures in which acoustic anisotropy of the test material presents difficulties in interpretation of the inspection results.
2. Background Art
Modern technological advances have placed increasing demands on the performance of superstructure buildings which contain sophisticated data communications equipment. Such buildings need to retain structural integrity under much more severe loading conditions, including seismic conditions, than conventional structures. Increasing numbers of modern buildings utilize super high strength construction steels, and the builders resort to ultrasonic technique to inspect the integrity of welded joints. Therefore, the efficiency of ultrasonic inspection procedure directly affects the productivity of such construction projects.
Typical of materials for buildings are high strength steels made by thermo-mechanical control processes (known as TMCP steels). These steels are produced by rolling at temperatures lower than those used for normal hot rolling steels, thus producing steels having a high acoustic anisotropy. In such steels, the velocity of propagation of the ultrasonic beam is different in the longitudinal and transverse directions. It is difficult to apply ultrasonic inspection techniques to such steels, because the propagation speed differences in the two directions contribute to misinterpretation of ultrasonic inspection results caused by false refractions. Therefore, there has been a long outstanding need for a weld inspection methodology and apparatus which would enable quick and accurate checking of welds in such TMCP steels.
Weld inspection procedures for acoustically anisotropic steels are standardized in the Japanese Industrial standard (JIS) No. Z 3060, hereinafter referred to as JIS, entitled "Methods of Manual Ultraionic Examination and Classification of Test Results for Ferritic Steel Welds". Ultrasonic inspection techniques are also defined in another standards, "Standards for the Ultrasonic Inspection of Weld Defects in Steel Structures", published by the Architectural Institute of Japan, hereinafter shortened to AIJ.
In the JIS, a transverse beam normal probe, which produces through-thickness travelling beams (hereinafter referred to as vertically-acting), to measure the velocity (C.sub.sl) in the primary rolling direction (i.e. in the longitudinal direction), and the probe then measures a transverse beam velocity (C.sub.sc), from which a sound velocity ratio (C.sub.sl /C.sub.sc) is calculated to determine whether there is acoustic anisotropy. A material is considered to have acoustic anisotropy when this ratio exceeds 1.02. Next, the angles of refraction data of the material, obtained with the use of a pair of angle-probes according to a so-called "V through scan method", are corrected by using the measured value of acoustic anisotropy and the standard test block (STB). There are regulations concerning the calibration of the measuring apparatus with a comparative testing block (RB-4), as well as the use of probes having a 60 degree refraction angle.
The AIJ regulation also mandates the use of the STB and the sound velocity ratio to correct the measured angles of refraction of the specimen.
These methods for the TMCP steels are much more complicated and time consuming than for the regular steels, because they are based on a two step process of: first determining the degree of acoustic anisotropy followed by the corrections of measured angles of refraction. Such requirements for the testing protocol adversely affected the efficiency of inspection, and ultimately added to the overall cost of construction projects which utilized TMCP steels.
Further practical problems with the existing method of determining the acoustic anisotropy are explained below.
In the measurements of the sound velocity ratio C.sub.sl /C.sub.sc from which acoustic anisotropy is obtained, it is necessary to measure the values of C.sub.sl and C.sub.sc very accurately. This operation required excessive time, and contributed greatly to lengthening of the testing period, and the consequent loss in construction productivity.
Further, the JIS requires the use of a comparative testing block (RB-4) to calibrate the testing apparatus, and this requirement was extremely difficult to be met in an environment of on-site construction activities. This further added to the cost of testing.
Further, the testing standards specify the use of either a 60 or 65 degree probe, irrespective of the degree of acoustic anisotropy of the construction material. Although construction materials come in a large variety of different shapes (for example, H-beams, rectangular beams and pipes) and their angles of bend and thicknesses have to be taken into account individually in order to obtain best results, there is no allowance for using probes having other angles.
The most serious drawback of the present method is that, in spite of the wide recognition of the importance of such testing, the method of acoustic anisotropy determination is being utilized only in large construction projects because the current methodology does not appeal to general construction industries.